What are referred to as electric firing circuits form the essential component of a controller for vehicle-occupant restraint systems, for example airbag controllers. Such firing circuits contain, as their essential component, at least one firing cap which causes the airbag propellant to explode when firing occurs by supplying electrical energy. Further components or at least two electronic switches which are frequently implemented as part of a customized circuit, for example in the form of a MOS field-effect transistor. The switches which are usually opened are closed when the respective airbag is fired, by means of a control signal which is correspondingly adapted by a driver circuit for controlling the switches (MOS field-effect transistor). The electronic switches have, in the connected-through state, an electrical contact resistance which is, inter alia, a function of the gate voltage of the respectively used MOS field-effect transistor. In order to achieve low power losses it is favorable to keep the contact resistance during the firing to a minimum. However, for this purpose the gate voltage of the MOS field-effect transistor must be as high as possible. This factor in itself requires the supply voltage of the driver circuit for the electrical circuits to be kept high for as long as possible, the supply voltage then being several volts above the firing voltage.
For this reason, a switching regulator which increases the battery voltage of the vehicle to the level of the firing voltage is usually provided in the airbag controller. A step-up converter, which has a capacitor (preferably electrolytic capacitor) for smoothing the switching voltage and for buffering the energy necessary to operate a control device (ECU), is usually used to step up the voltage. The energy reserve stored in the capacitor (acting as an energy store) is fed via a diode to a charge pump whose output voltage is several volts higher than the firing voltage. The voltage which is stepped up by the charge pump is then used to supply the driver circuit for the switches.
However, energy is removed from the energy store (capacitor) during the firing process and, as a result, the voltage across the energy store drops. This can lead to a situation in which sufficient voltage to ensure complete through-connection of the MOS field-effect transistors is no longer available. As a result of this, under certain circumstances the firing process can in turn only be carried out incompletely, which itself can lead to faults in the overall functioning of the airbag.